The present invention relates to a method for determining the radial modulus of elasticity of paper or a corresponding web-like material that can be reeled or wound on a reel. In said method the connection between the force and deflection of a material arranged in superimposed layers is measured.
A known method for measuring the radial modulus of elasticity of paper is a measurement conducted in a laboratory, which is disclosed for example in the publication by D. Roisum: The Mechanics of Winding, Tappi Press 1994, p. 62. The measurement is conducted in such a manner that a stack of paper sheets is pressed between two planes. As a result of the measurement a curve is attained, which represents the pressing force as a function of the height of the stack. The stress is obtained by dividing the force by the measurement area. The strain of the paper stack, which in this case is, in fact, compression, is obtained by dividing the change in the height by the original height of the stack. The paper stack is loaded until it reaches such stress which is substantially the same as the maximum stress that is assumed to be effective inside the reel. The loading of the paper stack is conducted several times in succession. The radial modulus of elasticity is the slope of the tangent of the stress-strain curve.
It is a problem of the laboratory measurement that it is conducted with a delay, in other words reactions to problems in the production occur slowly. The shape of the paper stack does not entirely correspond to the shape of the reel in the production machine. Furthermore, it is necessary to use a paper stack, wherein it is somewhat difficult to prepare a sample for the measurement. In this measurement it is, however, necessary to use a paper stack, because it is very difficult to measure single sheets and it may cause inaccurate results.